The present invention concerns a roller arrangement for a product to be coated in a molten metal bath, having a roller which rotates in the metal bath and is retained at the ends rotatably on support arms by way of rolling bearings, the rolling bearings being arranged in bearing housings sealed with respect to the metal bath.
In hot galvanizing, tinning, or aluminizing, one or more rollers, bearing-mounted at both ends, are immersed into a metal bath in order to guide therein the strip that is to be coated. Plain bearings have hitherto predominantly been used to support the rollers in the metal bath. These bearings, completely immersed in the metal bath, have the disadvantage of short service lives, since high temperatures, friction, and attack by the liquid metal cause severe wear on the bearing surfaces. The wear results in rough running or even brief seizing of the roller. The reason for the rough running or seizing of the roller is friction at the roller journal, which scores into the bearing shell as the diameter is reduced. Solid particles and surface damage in the contact region accelerate this process.
U.S. Pat. No. 5,571,328 and EP 0 524 851 A1 disclose plain bearings that use ceramic plates in a wedge-shaped arrangement as the outer bearing shell. This is intended to prevent the roller journal from scoring into the bearing shell. Wear-related surface damage and particle deposits on the bearing journal nevertheless subject the ceramic plates to impacts, and can cause premature bearing failure.
DE Patent 43 07 282 A1 discloses a further roller arrangement in which the buoyancy of an air-filled roller in the metal bath is used to reduce load. Rolling bearings whose rolling elements rotate in the liquid metal are provided to support the roller. A disadvantage of this embodiment, however, is the fact that as a result of wear and dissolution effects in the metal bath, internal bearing clearance increases and dimensional accuracy is lost.
The publication xe2x80x9cAntriebstechnikxe2x80x9d 34 (1995) No. 7 furthermore describes a rolling bearing for use in a molten zinc bath which is embodied as a solid ceramic bearing. A disadvantage of this embodiment, however, is the fact that during service in molten metal, solid particles can penetrate into the raceways, and subject the ceramic bearing to impacts as they are rolled over. In addition, the ceramic can suffer damage due to impacts or thermal shock while being installed or removed.
To solve this problem, DE Unexamined Patent Application 39 40 890 A1 proposes to place the bearings in closed bearing housings that are sealed with respect to the molten bath. Sealing is accomplished by way of a conical sleeve that is retained displaceably on a shaft and is pressed by a spring element against a conical trough secured to the bearing housing. During operation, friction associated with abrasion occurs between the rotating roller and the stationary portion of the conical seal, so that the arrangement can become leaky after extended operation.
It is therefore the object of the invention to develop a roller arrangement of the kind cited initially in such a way that even in extended operation, complete sealing of the bearing housing with respect to the metal bath can be guaranteed.
According to the present invention, this object is achieved substantially in that, in order to seal the bearing housing with respect to the roller, sealing surfaces extending transversely to the roller axis are configured on a radially projecting shoulder of the roller and on the bearing housing, and are pressed against one another or against an interposed seal element. The result of this configuration is that molten metal from the metal bath must pass between the sealing surfaces of the roller shoulder and bearing housing in order to get into the interior of the bearing housing. The gap formed between the sealing surfaces can be very well sealed by axially acting contact pressure, however, so that any penetration of molten metal into the bearing housing can be reliably prevented even during extended operation.
According to an embodiment of the invention, provision is made for the radial shoulder to be provided on the end portion of the roller engaging into the bearing housing, i.e. sealing occurs on the inner side of the bearing housing. In this case a gas conduit preferably opens into the bearing housing in order to deliver gas to the bearing housing and generate a positive pressure, and by way of the positive pressure to push the radial roller shoulder toward the sealing surface of the bearing housing. With this configuration, generation of the contact force in the region of the sealing surfaces is achieved by the fact that the internal pressure acting in all directions attempts to push the roller journal out of the bearing housing against the hydrostatic pressure of the metal bath. This force can be calculated as the product of the difference between the internal pressure and the hydrostatic pressure times the cross-sectional area of the roller journal in the passthrough region. This pneumatic contact force has the advantage of not requiring any additional installation space, and guarantees that the sealing surfaces touch one another and that the roller journal remains axially in position. Wear in the region of the sealing surfaces results in axial shifting of the roller journal. The running surfaces of the bearing must correspondingly allow axial displacement of the roller journal.
If the internal pressure is greater than the hydrostatic pressure in the molten metal, it constitutes an additional safety feature. The reason is that if leaks occur in the region of the sealing surfaces, for example as a result of wear, the result of the internal pressure is that molten metal does not enter, but rather gas emerges, at a volume flow that is kept low by the fact that the annular gap between roller and bearing housing is narrow.
According to a development, provision is made for there to be arranged, between the sealing surfaces of the roller shoulder and bearing housing, an annular disk made of seal material that is pressed via the roller shoulder against the sealing surfaces on the bearing housing. This embodiment has the advantage that the annular disk can be configured as a wear-and-tear part, and can easily be replaced as necessary. The annular disk can be joined to the roller so that it co-rotates with the roller. In this case wear occurs only between the annular disk and the bearing housing. Alternatively, it is possible to join the annular disk to the bearing housing, so that the annular disk is stationary and wear thus occurs only between the annular disk and roller shoulder. According to a preferred embodiment, however, the annular disk is coupled, for example by way of driver pins, to the cage of a rolling bearing, the result being that it co-rotates during operation and a uniform load thus occurs on the two sealing surfaces.
The annular disk is preferably made of ceramic, which has the advantage that it is inert with respect to the molten metal. The friction between the very low-roughness ceramic surface and the seal participant is largely wear-free. The scuffing that is possible in the case of contact between two metal surfaces can thus be eliminated.
In an alternative embodiment of the invention, provision is made for the sealing surfaces of the roller shoulder and bearing housing to be provided on both sides of the annular gap formed between the bearing housing and roller, and for a seal element to be provided that is simultaneously pressed against both sealing surfaces and seals the annular gap, the seal element being coupled to the rolling bearing cage so that during operation, the seal element rotates both with respect to the roller journal and with respect to the bearing housing. With this embodiment, therefore, the sealing surfaces on the bearing housing and roller are not pressed against one another as in the embodiments described above, but rather the two sealing surfaces each seal the annular gap on one side (radially inward and radially outward) by their coaction with the seal element. The result of the coupling with the cage of the rolling bearing is that relative motions occur both between the seal element and the bearing housing and between the seal element and roller, so that uneven abrasion at the sealing surfaces can be largely avoided.
The compressive force necessary for contact pressure on the seal element can be generated, for example, by spring elements. It is furthermore possible to generate in the interior of the bearing housing a positive pressure that, with the further advantages already explained above, additionally presses the seal element against the sealing surfaces.
Thermally related axial expansions of the roller, and the increase in the distance between the two bearings associated therewith, can be compensated for, for example, by adapting the transverse span width. According to an embodiment of the invention, for this purpose one of the support arms is pivotably suspended on the crossmember, and furthermore the outer side of the bearing housing is of partially spherical configuration and is arranged in a receiving shell, of complementary configuration, of the support arm. The ball-and-socket type of fit then compensates for the pivoting motion of the support arm, and ensures that the two bearings are always aligned with the roller. Thermal distortions of the support arms are also compensated for in this fashion.
The seating surface of the receiving shell for the partially spherical bearing housing is of conforming configuration with respect thereto. By way of the conforming fit, the operating force is directed from the bearing housing onto the receiving shell and into the support arm. The fit is configured in such a way that the operating force from the bearing housing is received in planar fashion, within a solid angle range, by the receiving shell, and the bearing housing is secured without distortion in its seat. The receiving shell is advantageously subdivided into several, for example three, segments. Rotation of the bearing housing in the receiving shell is prevented, so as not to damage the pressure input line to the chamber.
According to a further embodiment, provision is made for the bearing housing to be arranged in axially displaceable fashion in a receiving shell. This makes it possible to compensate for different degrees of thermal expansion. A pivotable support arm such as is provided in the case of the embodiment described above can, but need not, be present.
The bearing housing should possess a certain freedom of movement within the receiving shell, so that it is immobilized in the seating surface only by the operating force. This positioning should not occur until the thermal equalization processes in the roller and in the support arms have stabilized. This ensures that during operation, the orientation of the bearings with respect to the roller stays in alignment.
According to a further embodiment of the invention, the contact force of the contacting seal can be additionally increased by the fact that the pivotably suspended support arm is pressed outward with an external force, as a result of which the sealing surfaces of the bearing housing are pressed against the sealing surfaces of the roller shoulder. This external force is independent of the internal pressure, and may be necessary in the event of a brief drop in internal pressure.